1. Field of the Invention
The present invention relates to a device for post-processing exhaust gas from a diesel engine, more particularly to a device for discharging exhaust gas from a diesel engine, which improves a capacity for purifying exhaust gas from a diesel engine, especially capacity for purifying nitrogen oxides, by using hydrogen gas generated in an ammonolysis module.
2. Description of the Related Art
Generally, a diesel engine that has inherently high heat efficiency (i.e. high fuel economy) and high torque at a low speed, is useful for a device providing driving force to a vehicle, and may be driven at a high A/F (air to fuel) ratio under a lean condition of fuel mixture. For this reason, the diesel engine exhibits high HC and CO conversion ratios, but exhaust gas from the diesel engine discharges NOx and particulates highly. Both the NOx (nitrogen oxides) and particulates are components of diesel exhaust gas that are difficult to convert into harmless material. Nevertheless, the emission standards related thereto are increasingly restricted because of associated environmental problems.
In order to reduce particulates, methods for post-processing them are being focused upon a catalyzed diesel particulate filter (CDPF or DPF) also called catalyzed soot filter (CSF). Broadly speaking, these filters are constituted to capture and recycle particulates from exhaust gas. First in order to capture particulates, various filter systems applicable for absorbing particulates from exhaust gas of a diesel engine have been disclosed and includes honeycomb wall-flow filter and the like (See FIG. 1). The resulting particulates should be combusted (recycled) in the filter continuously or periodically, after being captured and accumulated within a filter. Therefore, catalysts are provided for the filter in order to decrease a temperature of combustion and also made to a catalyzed diesel particulate filter. Usually, the CDPF or CSF can decrease 90% or more of particulates as well as give passive combustion of particulates after being accumulated and filter recycling thereafter. In the DPF, carbon particulates can be combusted at 550° C., but oxidized even at 250° C. under the presence of NOx. Therefore, in order to produce NOx and lower the temperature of recycling, precious metals including Pt and/or Pd are loaded on the DPF since they oxidize nitrogen oxides to NO2 in exhaust gas discharged from a diesel engine. Accordingly, an additional module generally called diesel oxidation catalyst (DOC) may be also installed at the front end. When passing through the DOC, the resulting gas discharged from a diesel engine can generate NO2 as a by-product that can oxidize HC (hydrocarbon) and CO and coincidentally decrease the temperature of combustion in a DPF installed at the posterior end.
For such a filter recycling, carbon particulates can be oxidized at a relatively low temperature, for example at 200° C. or less, when hydrogen gas is sprayed at the front end of the CDPF or CSF additionally. In the meantime, when hydrogen is injected at the front end of the DOC, DOC performance is also improved due to decrease of LOT and further, desulfuration efficiency becomes better.
As disclosed in prior arts, various methods have been attempted to remove nitrogen oxides from exhaust gas of diesel engines, for example catalytic systems including selective catalytic reduction (SCR) or lean NOx trap (LNT). Especially in SCR systems, a urea-SCR(NH3-SCR) module produces harmless nitrogen components by a process comprising steps of: spraying urea within a storage container (tank) installed on a transport means toward the upstream of discharge gas; then, converting the urea to ammonia by using a hydrolytic catalyst; and reducing nitrogen oxides (NO, NO2) by applying the ammonia as a reducing agent. FIG. 2 shows an outlined diagram of the system for post-processing exhaust gas which comprises a urea-SCR module. FIG. 3 shows a detailed diagram of a urea-SCR module.
In addition, a so-called HC-SCR module is disclosed. Particularly, it is a catalytic converter that lessons NOx selectively by HC (hydrocarbon) as a reducing agent and the HC reacts with NOx selectively to produce N2, CO2, and water (H2O). Furthermore, in recent studies, it is elucidated that the selective reduction is improved highly when hydrogen is sprayed at the front end of a HC-SCR module.
Moreover, LNT (lean NOx trap) is known as a system for reducing nitrogen oxides. It is embodied with a mechanism that traps nitrogen oxides at a lean area and releases them at a rich area so as to convert them to nitrogen and carbon dioxide by using catalysts. This NOx trap is advantageous to have a good durability at a high temperature. However, it is problematic to poison by sulfur components within fuel and have a low recycling efficiency by HC and CO when being recycled at a rich spike.